FIG. 1A shows a perspective view of a hard disk drive 100. The hard disk drive 100 includes a data storage medium 180 and an actuating arm 182 which may be movable over the top surface of the storage medium 180. The actuating arm 182 includes a slider 184 arranged towards the tip region of the actuating arm 182. The slider 184 includes a head (e.g. a read/write head, or R/W head) 186 which may include a reader (or read head) and a writer (or write head). The actuating arm 182 may be controlled to position the head 186 over a track (e.g. data track) of the data storage medium 180 to read data from the data storage medium 180 by means of the reader or to write data to the data storage medium 180 by means of the writer.
State of the art head technology, as shown in FIG. 1B, uses a single sensor 191 as the read head, as part of a R/W head (e.g. 186) to read back data stored in a medium (e.g. 180). The read head (R1) 191 may be arranged between a pair of hard bias magnets 192a, 192b. The read head 191 and the hard bias (HB) magnets 192a, 192b may be arranged between a pair of main shield elements: main shield 1 193a, and main shield 2 193b. The arrows indicating “down track direction” and the “cross track direction” represent the arrangement of the read head 191 relative to the tracks of the medium. The parameter “V” shown in FIG. 1B represents the voltage measured between the two shields (main shield 1 193a and main shield 2 193b) connecting the sensor (R1) 191.
FIG. 1C shows a schematic top view of the prior art read head 191 of FIG. 1B relative to tracks of a data storage medium, illustrating a conventional reading scheme with a single giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) sensor to show an offset effect. For illustration purposes, four adjacent tracks 195a (Track 1; T1), 195b (Track 2; T2), 195c, 195d of the data storage medium are shown in FIG. 1C.
An essential requirement of the read sensor 191 is that its width, RW, should be smaller than the track width, TW, to avoid reading signal from adjacent tracks (e.g. 195a, 195b). Typically, the physical width, RW, of the sensor 191 must be about 50-70% of the track width, TW. Generally, the sensor, as indicated by 191a, should be located exactly at the center of the track (e.g. Track 1 195a) to reduce the read error. Any offset of the sensor will induce the loss of signal to noise ratio (SNR). As shown in FIG. 1C, when the sensor, as indicated by 191b, is offset towards Track 2 195b, the sensor 191b will also read signal from the Track 2 195b. The output or total voltage from the reader 191, being the reading signal, will be Vs=Va+Vb=aT1+bT2, where Va and Vb are related to the sensor response to Track 1 195a and Track 2 195b, respectively. The parameter Vs may be equivalent to the parameter “V” as described above in relation to FIG. 1B. The SNR is equivalent to (aT1/bT2) and will be reduced as 20 log (aT1/bT2). The state of the art servo system controls the offset so as to minimize bT2.
Therefore, the focus has been on the avoidance of reading from adjacent tracks. One option is to scale down the read width (RW), to be less than the track width (TW), i.e. RW<<TW. Another option is to reduce the inter-track interference (ITI) by servo control.
However, as the track width shrinks to accommodate the storage density increment, the sensor width or read width must decrease accordingly. As a result, read sensor widths are being pushed below the 20 nm scale. The sensor width is currently defined by lithography (e.g. deep ultraviolet (DUV), electron beam (e-beam), etc.) and etching (argon (Ar) ion milling, reactive ion beam etching, etc.) processes. As the data storage industry is passing semiconductor industry in the minimum feature size required for read sensors, it is becoming more and more difficult to shrink the sensor widths. Therefore, for the fabrication process, the critical dimension (CD) may be less than that available in the semiconductor industry. Also, a small read width may result in instability.
Further, while the track width in the recording media can be significantly reduced through shingled writing or two dimensional recording technique, the read track resolution is mainly limited by the sensor width.